Kesterite solar cells have long occupied a frustrating position in renewable energy research. Built from Earth-abundant, non-toxic materials, they look ideal for scalable, sustainable photovoltaics — yet their real-world efficiency has lagged well behind competing thin-film technologies for years.
Now, researchers have pushed kesterite to a certified efficiency of 15.3%, a record for the technology. The breakthrough didn’t come from swapping in new materials. Instead, it came from rethinking a fundamental step in how these cells are manufactured.
A promising material held back by its own growth process
Kesterite — shorthand for Cu₂ZnSn(S,Se)₄, or CZTSSe — is built entirely from elements that are abundant in Earth’s crust and carry low toxicity. That combination gives it strong sustainability credentials that silicon and many competing thin-film materials simply can’t match.
Performance has always been the problem. Kesterite cells have trailed rivals like cadmium telluride (CdTe) and copper indium gallium selenide (CIGS) by a significant margin in power conversion efficiency, limiting their commercial appeal despite their environmental advantages.
A central culprit is the selenization step — a high-temperature process that converts a precursor film into the active semiconductor layer. It’s essential, but poorly understood and hard to control. When selenization goes wrong, the film develops a bilayer structure with horizontal grain boundaries running through it. Voids open up, unwanted secondary phases appear, and each defect gives electrons somewhere to get lost — dragging efficiency down before the cell ever sees sunlight.
Tracing the flaw to its origin
The new study, published in Nature Communications, pinpoints where the trouble starts. At the very beginning of selenization, selenium reacts preferentially at the back interface of the cell — the surface farthest from incoming light.
That early reaction produces a copper sulfide-selenide phase, written as Cu(S,Se), which has a low melting point. Once it forms, it sets off what the researchers describe as “reverse crystallization”: grains begin nucleating and growing from the bottom of the absorber layer upward, rather than being guided in a controlled direction. The result is the disordered, defect-rich structure that has plagued kesterite cells for years.
Crucially, this specific back-interface mechanism hadn’t been clearly identified before. Naming the root cause is itself a meaningful scientific contribution — you can’t fix what you haven’t found.
The thermal-decoupled selenization strategy
The team’s solution targets the problem at its source. Their thermal-decoupled selenization approach engineers a vertical selenium concentration gradient during the early stage of the process.
By carefully separating when and where selenium is supplied from the temperature range at which the damaging Cu(S,Se) phase forms, the researchers prevent that phase from ever getting the foothold it needs. The harmful back-interface reaction is suppressed before it can trigger reverse crystallization. What grows instead is a columnar grain structure — orderly, vertical crystals running top-down through the absorber layer.
This geometry matters enormously for device performance. Columnar grains give charge carriers a cleaner, more direct path to the electrodes, and fewer grain boundaries mean fewer opportunities for electrons and holes to recombine before contributing to current. Better carrier transport and lower recombination are among the most direct levers available for raising solar cell efficiency.
Record efficiency and what the numbers mean
The champion device produced in the lab reached a power conversion efficiency of 15.7%. The certified figure — verified by an independent testing body — came in at 15.3%.
That gap between lab and certified results is normal. Independent certification removes any possibility of measurement error or optimistic conditions skewing the number, and a certified result is the one the field actually takes seriously.
For kesterite, 15.3% is a significant milestone. The technology spent years stuck well below competing thin-film benchmarks, and this result moves it meaningfully closer to commercial relevance. The gain came entirely from process engineering — no new or exotic materials entered the picture, just a smarter way of handling the ones already in use.
Beyond kesterite: implications for other solar technologies
The researchers also demonstrated that the thermal-decoupled approach works for solution-processed CIGS solar cells — a well-established thin-film technology with its own crystallization challenges. That cross-applicability is worth noting. It suggests the insights here aren’t niche, and that understanding how to manage crystallization kinetics in copper-based chalcogenide semiconductors could inform development across a broad family of next-generation solar materials.
From a commercialization standpoint, process-level innovations like this one tend to be more scalable and cost-effective than material substitutions. Manufacturers can often adapt existing equipment; redesigning supply chains is far harder.
Still, significant work remains. Pushing efficiency higher, demonstrating long-term stability under real operating conditions, and scaling from small champion cells to full-size modules are all challenges the field now faces. How quickly the community addresses them will determine whether kesterite’s record-breaking moment becomes a genuine turning point.
If you want to learn more about this discovery, you can check the full study here: Wu, Z., Wei, H., Shao, Z. et al. Thermal-decoupled selenization enables kesterite solar cells with 15.3% certified efficiency. Nat Commun (2026). https://doi.org/10.1038/s41467-026-74180-z
Carlos is an engineer with strong expertise in technical and industrial topics. He previously worked at international companies such as Siemens and speaks Spanish, German, English, and Italian.
